In wavelength division multiplex (WDM) optical communication systems, a plurality of optical carrier signals each having a different wavelength are multiplexed as separate transmission channels onto a single optical fiber. Distributed feedback (DFB) semiconductor lasers are the light sources of choice for generating the carrier signals. Early designs contemplated that separate lasers, each with a different grating period (or pitch) corresponding to a different wavelength, would be assigned to each channel. However, as pointed out by W-T. Tsang in U.S. Pat. No. 5,606,573 issued on Feb. 25, 1997 (incorporated herein by reference), this approach requires that the grating pitch difference between lasers be on the order of 0.1 nm for nominally 1550 nm DFB lasers. Such tight control of pitch strains the capability of both holographic and contact printing techniques used to make the gratings. Accordingly, Tsang proposed a laser design in which the laser-to-laser grating pitch is constant, but the active stripe is oriented at an angle to the transmission axis. The angle is varied from laser to laser to generate lasers having different wavelength outputs. Keeping the grating pitch constant simplifies the manufacturing process in one respect, but this advantage is to some extent offset by the need to vary the orientation of the active stripe from laser to laser.
Others have proposed lasers in which the physical design is fixed and the output wavelength is tuned electrically. Thus, a plurality of lasers all having the same physical design (but different electrical inputs) could provide all of the carrier signals of a WDM system. Alternatively, a single laser could provide all of the carrier signals provided two principal requirements are met: first, the laser has to be tunable over the spectrum spanned by the system channels; and second, the laser has to be switchable between different wavelength outputs at relatively high speeds. For example, H. Nakajima et al., OFC Technical Digest, p. 276, Paper ThQ5 (Feb. 1996; incorporated herein by reference) report a buried ridge structure DFB laser comprising two DFB gain sections separated by an intracavity Franz-Keldysh (F-K) electroabsorption bulk section. The latter provides wavelength control by an applied reverse bias voltage. The authors report that the laser was tuned at relatively high speeds but only over a relatively narrow wavelength range of 0.2 nm. In contrast, H. Hillmer et al., IEEE J. Selected Topics in Quantum Electronics, Vol. 1, No. 2,pp. 356-362 (1995; incorporated herein by reference), report a DFB multiquantum well (MQW) laser comprising a chirped grating and a bent active waveguide having a tilted half-sine shape. Three electrodes apply three separate currents to different sections of the active waveguide in order to tune the wavelength of the output. Compared to the Nakajima et al. laser, an order of magnitude improvement in tuning range (5.5 nm) was demonstrated but only at relatively slow speeds (i.e., at DC). One would expect the tuning range at high speeds to be considerably less.
However, some optical systems (e.g., packet switching systems) require relatively high speed tuning over a broader range (e.g., several nm) , whereas other systems (e.g., WDM transmission systems) require less speed but demand a much broader tuning range (e.g., 10-12 nm).